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Planetary aurorae trace an interplanetary shock from the Sun to Saturn (9 AU)
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Planetary aurorae trace an
interplanetary shock from the
Sun to Saturn (9 AU)
Date de mise en ligne : mercredi 1er décembre 2004
Observatoire de Paris centre de recherche et enseignement en astronomie et
astrophysique relevant du Ministère de l'Enseignement supérieur et de la
Recherche.
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Planetary aurorae trace an interplanetary shock from the Sun to Saturn (9 AU)
Astronomers from Paris Observatory (LESIA) observed for the first time a magnetic storm
in the Saturn magnetosphere by analyzing UV aurorae of this planet, seen by the Hubble
Space Telescope Hubble. Thanks to a favorable geometrical configuration of planets, they
showed, with the help of a numerical code for solar wind propagation (written at Michigan
University), that it is the auroral response of Saturn to the impact of an interplanetary shock
on the Saturn magnetosphere. On its passage, this shock, due to a series of solar coronal
plasma ejections, had before excited magnetic storms on the Earth and Jupiter. These results
were published in Nature last month.
One knows since the beginning of spatial era that the relation between solar activity and the extent and intensity of
polar aurorae on Earth comes from the solar wind plasma and magnetic field. The interaction between solar wind
and the magnetosphere occurs at the border between the closed lines of terrestrial magnetic field and those which
are open on the interplanetary medium. This interaction takes the form of magnetic reconnections between the
terrestrial and solar fields. From that results a magnetospheric activity - and thus an auroral activity - permanent, of
relatively low level, located primarily on the night side. When the passage of an interplanetary shock compresses the
magnetosphere brutally, the equilibrium of the magnetospheric plasma is affected, and precipitations of energetical
charged particles grow in intensity and extent towards the low latitudes : it is a geomagnetic storm accompanied of
very bright and very wide aurorae. One also knows since the Voyager missions the existence of permanent aurorae
on the (magnetized) planets Jupiter, Saturn, Uranus and Neptune, and one knows that they are strongly correlated
with the fluctuations of speed, density and pressure of the solar wind. But it had never been possible until now to
observe directly the equivalent of a geomagnetic storm in response to the interplanetary passage. Thus it was not
known whether a shock keeps intact its properties of interaction with the magnetospheres during its propagation
through the solar system, and whether the nature of interaction is the same for all magnetized planets.
Figure 1
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Planetary aurorae trace an interplanetary shock from the Sun to Saturn (9 AU)
Since 1992 for Jupiter, and 1996 for Saturn, R. Prangé and L Pallier observe the planetary aurorae with the Hubble
Space Telescope (HST). The images taken in far UV with very high space resolution (1 pixel 75 km on the Jupiter
disc and 150 km on that of Saturn) allow a very precise study of the morphology of the aurorae and its variability.
The Saturn aurorae obtained during several years appeared rather simple, consisting in a bright ring around each
pole (as on the Earth) with an asymmetry morning-evening in latitude and intensity. December 7, 2000 on the other
hand, this ring, more intense than usual, is accompanied by an active region, very bright at0 high latitudes on the
polar cap. The following day, all returned back to order (figure 1). One then suspects that it is the first observation of
a magnetic storm on Saturn. But no space probe is then in the immediate vicinity of Saturn there to measure the
solar wind and to confirm this assumption.
Découverte d'un orage auroral (ou magnétique) sur Saturne le 7 décembre 2000 à 11:30 TU (Ã gauche). Les
aurores ont retrouvé leur aspect usuel le 8 décembre 2000 à 10:00TU (Ã droite). Images en fausses
couleurs prises par le Space Telescope Imaging Spectrograph (STIS) à bord de HST dans les bandes de
Werner et de Lyman de H2 ( 130 - 165 nm) excitées par collisions avec les particules énergétiques
précipitées de la magnétosphère sur l'atmosphère. Le disque est vu par réflexion du continu solaire dans la
bande spectrale. Un sphéroïde aplati et un système de coordonnées ont été ajustés au limbe visible de la
planète.
Helped by an American colleague who develops a magnetohydrodynamic code (MHD) of solar wind propagation
from Earth until several AU, the LESIA team benefits from the exceptional planetary conjunction at the time of these
observations : the Sun, the Earth, Jupiter and Saturn then were almost aligned, and measurements of the solar wind
parameters were realized a little upstream of Jupiter by the Cassini probe (on the way towards Saturn). The same
solar wind thus met successively three planets, as shown on the graph at left, and the validity of the MHD model
could be tested by comparing its extrapolations with the measurements made by Cassini within 5 AU of the Sun. The
conclusions are summarized by figure 2 : a series of solar coronal matter ejections (CME) observed by SOHO
generated a series of shocks in solar wind, measured two days later by WIND and ACE close to the Earth. At the
Jupiter distance, after 15 days of propagation, these shocks amalgamate in a wider shock (as simulated by the MHD
code and measured by Cassini). One month later, this shock reaches Saturn at the time of the violent auroral
disturbance, whose magnetic storm nature is thus shown. The left-hand column of figure 2 also shows that
magnetic storms were started on each three planet successively at the time of interaction of this shock with their
magnetosphere. Finally, one notes resemblances between the auroral answers of the Earth and Saturn
(intensification of the aurorae on the night side, activation of open lines of magnetic field), but also differences (no
expansion of the auroral oval on Saturn, and asymmetry morning-evening) which carry the signature of coupling
processes specific to each planet. We now expect, from the Cassini mission in orbit around Saturn, a harvest of
new data which will allow to understand more completely the interactions solar wind/planet magnetospheres at the
scale of the solar system.
Reference
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Planetary aurorae trace an interplanetary shock from the Sun to Saturn (9 AU)
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An interplanetary shock traced by planetary auroral storms from the Sun to Saturn Renée Prangé, Laurent
Pallier, Kenneth C. Hansen, Russ Howard, Angelos Vourlidas, Régis Courtin & Chris Parkinson Nature 432, 78
(2004) 4 November
Contact
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Renée Prangé (Observatoire de Paris, LESIA)
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